Blowing heads of this type have been proposed in different designs, the internal cooling air supplied and removed in the axial direction through the film blowing head serves for the cooling of the extruded tubular film and for the inflation of the tubular film to form a tubular film bladder stretching out the fused synthetic resin tubular film. The external cooling ring has an annular nozzle gap directed towards the extruded tubular film gap, through which the cooling air is caused to impinge on the film so that the film is rapidly cooled down with the lowest possible so-called frost line in order to increase the output rate.
In order to be able to control the thickness of the tubular film, inflated to constitute a tubular film bladder, around its periphery, it is possible, in accordance with the prior art, to heat and/or to cool the tubular extruded fused synthetic resin differentially in sections of the periphery thereof in order to influence thickness gradient. In this case the effect is profited from that on inflating the tubular synthetic resin film to constitute a film bladder the hotter parts and the cooler parts are more and, respectively, less extended.
In the case of a film blowing head in accordance with the German patent 2,658,518 of the type initially mentioned for influencing the extruded tubular film to be at different peripheral temperatures a plurality of correcting air nozzles is arranged over the external air blowing ring in a circle encircling the tubular film, adjustment valves, which are suitably controlled, being associated with the individual correcting air nozzles. However this known film blowing head is comparatively involved in design owing to the necessity of having a plurality of radial air nozzles which are pointed towards the extruded tubular film to blow air at different temperatures onto it.